Metro apps will share the same runtime, so developers can create one app to run on all Windows devices

Microsoft today announced its Universal Windows Apps technology, which is intended to streamline the process of building applications that can run on Windows Phones, Xbox game consoles, and the Metro environment in Intel-based Windows 8.1 tablets and PCs. Previously, developers had to create separate apps for the forthcoming Windows Phone 8.1, for Xbox, and for the tablet/PC Metro, which made it less attractive for developers given the relatively small size of those two markets. Now, they can address all three environments with a single app.

Universal Windows Apps is based on the Windows Runtime and will be enabled through the Visual Studio Update 2 released today. Essentially, Microsoft is updating the Windows Runtime on Windows Phone 8.1 and Xbox to be the same as the Windows Runtime in Windows 8.1. That means they will use common APIs for everything from security to notifications, and the applications' user interface will be common across the environments.

"Universal Windows Apps removes some barriers of developing applications that work on both Windows Phone and the [Metro] environment of Windows 8.1," says Rob Sanfilippo, an analyst at Directions on Microsoft. He considers this technology a game-changer for Microsoft developers.

The universal apps will be available in a converged Microsoft Store that serves Windows Phone 8.1, Xbox, and Windows 8.1 users.

Microsoft's Universal Windows Apps concept and its potential time-savings for developers sounds good, provided it works, said developer Radek Voltr, CTO of client development at business solutions company Xacti and an attendee at Build. "I think that's great, and it will be great for users."

Nokia plans to have Windows Phone 8.1 on existing Lumia devices this summer through over-the-air updates. The first new devices running Windows Phone 8.1 will be available this May in Europe.

Microsoft also announced that Windows will be available to manufacturers at no cost for smartphones and for tablets with screens smaller than nine inches, as an incentive to get more hardware makers to develop mobile Windows devices. Currently, Microsoft's newly acquired Nokia Devices Group supplies 80 percent of the world's Windows Phone devices, as manufacturers like HTC and Samsung have pulled back their Windows Phone efforts. Likewise, Microsoft's Surface Pro is the leading Windows 8 tablet, though there's a larger set of Windows tablet makers than there are Windows Phone makers.